Beamformers are known. Such devices may be used to direct radio frequency (RF) energy (emissions) to a specific target at a specific location. Such directed RF emissions ("transmit beamforming") may be accomplished through the use of directional antenna(s) or through the use of antenna arrays. Where antenna arrays have been used the characteristics of the RF emissions may be influenced by array element positioning or by a mathematical weighting of outputs from array elements.
While the process of transmit beamforming may not be difficult, the location to which an RF emission is to be directed may not be readily identifiable. Where the source is a radar transponder, the solution is simplified in that the operator simply selects the direction of transmission and waits for a response. Where, on the other hand, the target is a mobile communication unit then the situation may be considerably more difficult. Transmit beamforming relative to mobile communication units is typically based upon some type of locational feedback from the target.
Methodologies of maximizing a receive signal ("receive beamformers") are also known. Receive beamformers typically receive a signal from an antenna and, through a process of mathematical analysis (or select a set of receive characteristics, maximizing receive signal quality. Where the antenna is a directional antenna the antenna may simply sweep an arc (containing the target) seeking the point of maximum signal strength from a desired target.
Antenna arrays may also be configured as receive beamformers through adjustments to physical positioning of array elements, or through adaptive filtering. Changing the positioning of array elements, on the other hand, may lead to unexpected results and loss of signal integrity. Adjustments to positioning of array elements also interferes with reception of RF signals from outside a selected beam area.
In general, where signals must be simultaneously received from large numbers of geographically dispersed communication units, physical positioning of antenna element is not practical. Where physical positioning of antenna elements is not practical, receive beamforming may be performed through mathematical analysis of signals received through a multitude of antenna elements.
Where receive beamforming is performed through mathematical analysis, the beamformer may exist in a mathematical sense only and may be considered a subset of adaptive filtering (see Adaptive Filter Theory, 2nd ed., Simon Haykin, Prentice Hall, 1991). The receive beamformer, in such case, may be considered as a form of spatial filter attenuating all but selected signals. Since a set of input signals from an antenna array may be received and stored, any number of receive beamformers may operate upon a given set of stored data to produce any number of signals from stored input data.
A cellular radiotelephone system is an example of a situation where receive beamforming may be performed through adaptive filtering (adaptive beamforming). Adaptive beamforming in such a system is typically performed at a base site which includes an antenna array and through which a number of simultaneous communication transactions may occur.
Adaptive beamforming, in general, may be performed through calculation of a set of antenna array weights. The set of antenna array weights minimizing interference may be calculated using measurements from the array when both a known desired signal and interferers are present. The set of weights may then be used to cancel interference during periods when the desired signal is not known, provided that the location of the sources of interference and the desired signal remain substantially constant. The weights which minimize the interference may be calculated by solving the complex equation as follows: EQU Xw=y
The value, X, is a N.times.M matrix of array (signal) (simultaneously sampled array outputs), where N is the number of snapshots, and M is the number of antenna elements. ##EQU1## The value, y, is the N.times.1 vector of the (known) desired signal. ##EQU2## The value w is an adaptive array weight vector (M.times.1) for all array elements. ##EQU3## Given the weight vector, w, the adaptive output of the beamformer may be computed at any time, t: ##EQU4##
While receive beamformers have worked well, an antenna array is typically required as a prerequisite for receive beamforming. Portable communication units (because of size and weight limitations) are typically not equipped with antenna arrays.
An alternative to receive beamforming (at a portable) is transmit beamforming at a base site. Transmit beamforming at a base site may allow significant signal energy to be directed to the location of a portable without significantly interfering with reception by another portable.
Transmit beamforming, on the other hand, has proved difficult (in practice) because of the difficulty of determining transmit beamform array coefficients. Part of the difficulty of determining transmit coefficients lies in the fact that the coefficients of a receive beamform array used in beamforming a received signal have very little relationship to the coefficients of beamforming a transmitted signal. Phase differences and non-linearities in receive and transmit elements make receive beamform arrays inapplicable to beamforming a transmitted signal. Because of the importance of mobile communications a need exists for a simpler method of beamforming transmitted signals from base sites to portable communication units.